[PATCH] ARM: unwind: set frame.pc correctly for current-thread unwinding

[PATCH] ARM: unwind: set frame.pc correctly for current-thread unwinding

[Russell King (Oracle)]

When e.g. a WARN_ON() is encountered, we attempt to unwind the current
thread. To do this, we set frame.pc to unwind_backtrace, which means it
points at the beginning of the function. However, the rest of the state
is initialised from within the function, which means the function
prologue has already been run.

This can be confusing, and with a recent patch from Ard, can result in
the unwinder misbehaving if we want to be strict about the PC value.

If we correctly initialise the state so it is self-consistent (in other
words, set frame.pc to the location we are initialising it) then we
eliminate this confusion, and avoid possible future issues.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
---
 arch/arm/kernel/unwind.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/arm/kernel/unwind.c b/arch/arm/kernel/unwind.c
index a007af0f0209..0e2244e26f37 100644
--- a/arch/arm/kernel/unwind.c
+++ b/arch/arm/kernel/unwind.c
@@ -487,7 +487,12 @@ void unwind_backtrace(struct pt_regs *regs, struct task_struct *tsk,
 		frame.fp = (unsigned long)__builtin_frame_address(0);
 		frame.sp = current_stack_pointer;
 		frame.lr = (unsigned long)__builtin_return_address(0);
-		frame.pc = (unsigned long)unwind_backtrace;
+		/* We are saving the stack and execution state at this
+		 * point, so we should ensure that frame.pc is within
+		 * this block of code.
+		 */
+here:
+		frame.pc = (unsigned long)&&here;
 	} else {
 		/* task blocked in __switch_to */
 		frame.fp = thread_saved_fp(tsk);
-- 
2.30.2


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Re: [PATCH] ARM: unwind: set frame.pc correctly for current-thread unwinding

[Ard Biesheuvel]

On Wed, 9 Mar 2022 at 13:10, Russell King (Oracle)
<rmk+kernel@armlinux.org.uk> wrote:
>
> When e.g. a WARN_ON() is encountered, we attempt to unwind the current
> thread. To do this, we set frame.pc to unwind_backtrace, which means it
> points at the beginning of the function. However, the rest of the state
> is initialised from within the function, which means the function
> prologue has already been run.
>
> This can be confusing, and with a recent patch from Ard, can result in
> the unwinder misbehaving if we want to be strict about the PC value.
>
> If we correctly initialise the state so it is self-consistent (in other
> words, set frame.pc to the location we are initialising it) then we
> eliminate this confusion, and avoid possible future issues.
>
> Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>

We'll need something similar for __save_stack_trace(), and for
return_address() if we ever decide to re-enable it for
CONFIG_ARM_UNWIND=y




> ---
>  arch/arm/kernel/unwind.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm/kernel/unwind.c b/arch/arm/kernel/unwind.c
> index a007af0f0209..0e2244e26f37 100644
> --- a/arch/arm/kernel/unwind.c
> +++ b/arch/arm/kernel/unwind.c
> @@ -487,7 +487,12 @@ void unwind_backtrace(struct pt_regs *regs, struct task_struct *tsk,
>                 frame.fp = (unsigned long)__builtin_frame_address(0);
>                 frame.sp = current_stack_pointer;
>                 frame.lr = (unsigned long)__builtin_return_address(0);
> -               frame.pc = (unsigned long)unwind_backtrace;
> +               /* We are saving the stack and execution state at this
> +                * point, so we should ensure that frame.pc is within
> +                * this block of code.
> +                */
> +here:
> +               frame.pc = (unsigned long)&&here;
>         } else {
>                 /* task blocked in __switch_to */
>                 frame.fp = thread_saved_fp(tsk);
> --
> 2.30.2
>

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